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Scathophaga stercoraria
It was very small, about 6-8mm long.
According to Portugal's Biodiversity Database, there are 3 species of the Scathophaga genus in Portugal - litorea, lutarea and stercoraria. Stercoraria morphologically is the most similar and appears to be the most abundant/well-documented, so I guess we have a winner!
Thank you so much, Fyn.